Static Random Access Memories (SRAMs) are semiconductor elements widely applied in modern computing devices. For the efficiency of operation, a “self-timed” SRAM is developed, which controls timing independently of the externally generated clock signal.
In a self-timed SRAM design, in order to generate enough bit line (BL) differential voltage values for robustly reading out sensing results, a bitcell tracking design is applied, where a tracking bitline (TRKBL) is used to model the BL discharge delay of the accessed BL and then generate sense amplifier (SA) firing and word line (WL) falling edges.
However, the trip point variation across Process/Voltage/Temperature (PVT) causes a significant gate delay fluctuation on a tracking BL receiver (usually implemented by an inverter or a NAND gate), which makes the SA being fired too early at the slow-fast (SF) corner or too late at the slow-slow (SS) corner, and thus degrades the tracking ratio. The SF corner denotes slow n-channel metal-oxide-semiconductor field-effect transistors (n-channel MOSFETs, or NMOSs) and fast p-channel metal-oxide-semiconductor field-effect transistors (p-channel MOSFETs, or PMOSs), and the SS corner denotes slow NMOSs and slow PMOSs. The tracking ratio is a performance indicator defined as the read margin at WCL (Worst Case Low Temperature, or speed sign off corner) over the read margin at the worst case PVT. The larger the ratio indicates the more read margin is wasted at WCL in order to make the read margin at the worst case PVT able to overcome the offset of the trip point of the SA. Since the read margin is desired to be constant across PVT, the closer to 1 the ratio is, the better the compensation performance is.
In order to cover earlier SA firing cases at the SF corner, more margins have to be put on the wordline (WL) to SAE (SA enabling) delay, for instance. One way to add margins is adding a logic gate delay after the TRKBL receiver, but this damages SRAM access time and develop redundant read margin at slow corners, such as the SS corner.